A plating apparatus is a device for depositing a conductive material on a surface of a wafer by passing an electric current between an anode and the wafer which are immersed in a plating solution. During plating of the wafer, the wafer is held by a substrate holder, and the wafer, together with the substrate holder, is immersed in the plating solution. The wafer is coupled to a power source via the substrate holder, while the anode is coupled to the power source via an anode holder. The substrate holder has a plurality of electric contacts which are brought into contact with a peripheral portion of the wafer, so that the wafer and the substrate holder are electrically connected through these electric contacts. When a voltage is applied between the wafer and the anode, an electric current flows from the anode through the plating solution to the wafer, thereby causing a deposition of a conductive material on the surface of the wafer.
In order to uniformly deposit the conductive material on the wafer surface, it is necessary to appropriately control an electric field which is formed between the wafer and the anode. From this viewpoint, a regulation plate (or an electric-field shielding plate) is provided between the wafer and the anode. This regulation plate has a circular hole whose diameter is smaller than a diameter of the wafer, and regulates the electric field by allowing the electric current, flowing from the anode to the wafer, to pass only through the circular hole.
However, even if such regulation plate is provided, a larger amount of conductive material is deposited on the peripheral portion of the wafer, compared with other portion of the wafer. This is because the electric contacts of the substrate holder are arranged so as to touch the peripheral portion of the wafer. Such a non-uniform deposition of the conductive material may decrease yield of device products. Therefore, there is a demand for a technique that can uniformly deposit a conductive material on a surface of a wafer.
In addition, there is also a demand for an improved substrate holder which does not cause a contamination of a wafer. The substrate holder has a base member and a holding member which are configured to sandwich a wafer between them. The substrate holder includes a locking mechanism that locks the holding member to the base member and unlocks the holding member from the base member, so that the substrate holder can detachably hold the wafer. However, the locking mechanism necessarily has sliding parts, and therefore contact surfaces of these sliding parts may generate particles. When the substrate holder is pulled out of a plating solution after plating of the wafer, the plating solution containing the particles flows downwardly from the substrate holder onto a plated surface of the wafer, thus possibly contaminating the wafer.
Moreover, the substrate holder generally includes a component made of corrosion-resistant metal, such as titanium, which is highly resistant against a plating solution. The component made of such a metal may generate a by-product as a result of a reaction with the plating solution. When the substrate holder is pulled out of the plating solution after plating of the wafer, the plating solution containing the by-product flows downwardly from the substrate holder onto a plated surface of the wafer, thus possibly contaminating the wafer. Thus, there has been a demand for an improved substrate holder capable of preventing such a contamination of the plated surface of the wafer.